This invention relates generally to applicators for hot melt thermoplastic adhesives, and, more particularly, to a nozzle capable of dispensing hot melt adhesive in the form of a cellular foam.
Hot melt thermoplastic adhesives or so-called "hot melts" are widely used for adhering many diverse products. One of the most common applications of such adhesives is for packaging and cartoning where the quick setting time of hot melts is particularly advantageous.
Among the most common problems with hot melt adhesives are those of spreading the adhesive over the desired surface area and then compressing the adhesive after application so as to obtain sufficient surface contact between the adhesive and adhered substrate to achieve a good bond. The relatively high viscosity, high surface tension and quick setting time of hot melt adhesives can combine to prevent the adhesive from spreading over a large surface area when the adhesive is applied as a liquid to the substrate. Instead of spreading, the liquid adhesive sets up as a thick bead on the substrate. Even when quickly compressed, as for example between two flaps of a carton, the liquid adhesive is sometimes difficult to spread. It has been found that when two surfaces which have been adhered by liquid hot melt adhesive are pulled apart, the bond can break between the adhesive-substrate interface. Consequently, in order to achieve a good bond it is desirable to provide a large area of surface contact between the adhered parts.
As discussed in U.S. Pat. No. 4,059,714 to Scholl et al, which is assigned to the same assignee as this invention, improvements in spreadability and bonding strength are achieved with a given quantity of selected hot melt adhesive if the adhesive is applied as a cellular foam to the substrate rather than as a conventional non-foamed, liquid adhesive. The method of producing foam adhesive disclosed in Scholl et al involves the injection of air or a gas such as nitrogen into the liquid adhesive under high pressure in the range of 300 pounds per square inch. When the liquid adhesive/gas solution is subsequently dispensed into atmospheric pressure, the gas evolves from the solution in the form of small bubbles which become entrapped in the surrounding adhesive and cause it to expand volumetrically to form a homogeneous, closed cell, solid adhesive foam.
In the alternative, a similar foam may be formed as disclosed in U.S. Pat. No. 4,247,581 to Cobbs, Jr. et al, which is assigned to the same assignee as this invention. In the Cobbs method, a so-called blowing agent is placed into solution with the liquid adhesive under appropriate temperature and pressure conditions. When the solution is exposed to a pressure less than that required to maintain the blowing agent in solution, gas bubbles are produced and become entrapped in the liquid forming essentially the same type of closed cell solid adhesive foam as in the Scholl et al method.
Regardless of the manner in which closed cell, solid adhesive foam is produced, several advantages are provided as compared to conventional liquid hot melt adhesive. Foamed adhesive is much easier to spread along a substrate than unfoamed hot melt adhesive, which is attributable to the surface characteristics of foamed adhesive. In contrast to the viscous liquid adhesive, foamed adhesive is much less viscous allowing a greater volume of foamed adhesive in the molten state to be moved or flattened by a given force per unit of time than may be moved or flattened by the same force acting upon the same molten adhesive in an unfoamed state. In addition, whereas liquid adhesives are incompressible, foamed adhesives are compressible because of the presence of gas bubbles in the adhesive and thus easier to spread.
It has also been found that foamed adhesives have a longer "open" time during which they retain their bonding strength after being dispensed onto a substrate. Foamed adhesives also have proved to set up and adhere faster when compressed between two substrates, as for example two flaps of a carton. These two characteristics together are very desirable in cartoning applications because they eliminate the need to instantly close flaps after application of the adhesive and also permit the adhered surfaces to be released from clamping pressure after a short period of time.
The improved spreadability, bonding strength and rapid set up time of foamed adhesives, as compared to liquid hot melt adhesives, are achieved by the formation of a homogeneous solid foam having small, regularly spaced air or gas pockets throughout the adhesive. Problems arise with the formation of foamed adhesive having non-uniform gas bubbles including large bubbles which are not in solution in the adhesive. Large bubbles produce randomly spaced voids in the adhesive accompanied by small amounts of foamed adhesive in separate droplets. Such non-uniform foamed adhesive tends to sputter and spit when ejected from the foam producing system creating an uneven deposit of adhesive material on the substrate. Adhesive foam having non-uniform gas or air bubbles also becomes unstable more quickly and thus does not retain its bonding strength as long as desired after being dispensed onto a substrate. Poor foam quality and foam stability are particular problems in many prior adhesive foam guns when they are operated through an on/off/on cycle. In addition, many adhesive foam guns fail to provide foam instantaneously when cycled from off to on.